1. Field of the Invention
This invention relates to methods and apparatus for the spray casting of metallic preforms and, more particularly, to methods and means to shape a stream of gas-atomized metal particles deposited on a heated substrate thereby providing a monolithic product with minimized or no wastage due to thin metal deposits near the edges of the particle stream.
2. State of the Art
Sprayforming involves the feeding of molten metal through a nozzle into an enclosed chamber filled with inert gas. After exiting the nozzle, the molten metal stream is atomized by inert gas jets into a fine spray or plume of molten droplets which are projected onto a collecting surface. In the simplest cases, the collector is a flat disc that rotates to produce an even deposit. To make a solid cylinder, the collector plate is withdrawn by a hydraulic ram at the same rate as the top surface layer is being built up. To make metal matrix composites (MMCs), refractory, e.g. silicon carbide, powder is added during the atomization process to mix with the powder in the metal droplet stream.
A process for metal spraying, or metallizing, in which droplets of hot metallic materials are projected onto a base material to form a coating, was developed in Switzerland about 1910 by Dr. M. U. Schoop. Such metallizing differs from spray forming in that, in the former process, deposition rates are an order of magnitude less, and is used mainly for coatings. In the 1940s and 1950s, an early proponent of spray forming, Brennan, suggested various techniques for producing metal strips. In the 1960s, Professor Singer developed the so-called "Spray Rolling Process," a system to produce metal sheet by inert gas atomization and deposition of the thus-produced metal particles onto a rotating drum. Sprayforming Developments Ltd. (SDL), located at the Innovation Centre, University College, Swansea, Wales, is a company formed to exploit the developments of Singer and his co-workers and holds many patents, including those covering the basic sprayforming process, centrifugal spray deposition, simultaneous shot-peening, pneumatic scannning atomizer, multi-phase metals, coatings, and spray control; see, for example, Singer U.S. Pat. Nos. 4,224,356 and 4,515,864. See also Jenkins et al. "Melt Heat Extraction in the Sprayforming of Strip: The SDL Experimental Strip Unit."
The Osprey.TM. process was developed in the early 1970s, and involved preprogrammed control of a substrate motion to accept deposition of semi-solid metal droplets. A key point in this process is that the substrate surface also must remain semi-solid during the deposition process. Therefore, instead of a particulate microstructure (characteristic of thermal spraying techniques) a fine, uniform, equiaxed grain structure, with no interconnected porosity, is formed. Of particular significance is the ability of the Osprey.TM. process to operate at high deposition rates producing large billets with uniform microstructures or equivalent or superior properties to conventionally-produced materials. Osprey Metals Limited, Neath, Wales, also holds many patents; see, for example: Brooks U.S. Pat. No. 3,909,921; Coombs U.S. Pat. No. 4,779,802; Leatham et al. U.S. Pat. No. 4,804,034; Coombs et al. U.S. Pat. No. 4,905,899; Leatham et al. U.S. Pat. No. 4,938,275; Brooks et al. U.S. Pat. No. 4,926,924; Brooks et al. U.S. Pat. No. 4,926,923; Leatham et al. U.S. Pat. No. 5,110,631; Leatham et al. U.S. Pat. No. 5,143,139; Coombs et al. U.S. Pat. No. 5,196,049; Watson et al. U.S. Pat. No. 5,240,061, and Brooks Re 31,767; see also Leatham et al. "The Osprey Process: Principles and Applications," The International Journal of Powder Metallurgy, Vol. 29, No. 4 (1993) pages 321-329.
In 1980, Sandvik, in Sweden, developed a large-scale spray forming plant for the production of tubes and produces special grades of stainless steels and superalloys in tubular shape up to 8 meters long.
In 1985, Alcan, in Canada, under license from Osprey, installed a small facility producing sprayforms 300 mm diameter and 1 meter long, and patented some new aspects, including: twin head atomizers; a continuous production plant; Al-Li alloys; MMC particulate feeder/preheater; Ultralite alloys, and powder products from overspray.
In 1991, Sumitomo Heavy Industries, in Japan, installed a commercial spray forming facility to produce roll preforms. This facility has the ability to produce 1 ton high-carbon, high-speed steel rolls up to 80 mm in diameter and 500 mm long and having longer service life as compared to conventionally cast rolls, mainly as a result of a finer and more uniform carbide microstructure.
Various articles have been published concerning spray forming by one process or another. See for example: Eadie, "The Continuous Production of Steel and Alloy Strip Using Sprayforming," a British Steel publication; Payne et al., "Application of Neural Networks in Spray Forming Technology," The International Journal of Powder Metallurgy, Vol 29, No. 4 (1993), pages 345-351; Annavarapu et al., "Evolution of Microstructure in Spray Casting," The International Journal of Powder Metallurgy, Vol. 29, No. 4 (1993) pages 331-343; Tsao et al., "Modelling of the Liquid Dynamic Compaction Spray Process," The International Journal of Powder Metallurgy, Vol. 30, No. 3 (1994), pages 323-333.
In the sprayforming process, the atomized metal particles are deposited directly onto a stationary or moving substrate to form a monolithic product. See also Melillo et al. U.S. Pat. No. 5,143,140; Ashok et al. U.S. Pat. No. 5,131,451.
In most of these processes, atomization is carried out by the socalled remotely-coupled technique in which the atomizing gas hits the metal stream some distance away from its exit from the liquid metal reservoir. More recently, a closely-coupled technique has been used in the production of metal powder in which the atomizing gas is directed at the liquid metal stream just as it emerges from the liquid metal nozzle. This latter technique has the advantage of providing greater amounts of strengthening elements in solid solution and/or in the form of finely dispersed precipitates. Gigliotti, Jr. et al. U.S. Pat. No. 5,366,204. It is believed that the closely-coupled technique has not been used in commercial sprayforming.
Secondary jets have been used to cool the descending plume of atomized metal particles. U.S. Pat. No. 5,196,049 (secondary jets are positioned adjacent the primary, atomizing, jets for directing cooling fluid at the atomized droplets); U.S. Pat. No. 4,787,935 (high pressure cooling fluid is directed at the descending metal particle plume through passageways in the wall of a chamber in which the plume is formed).
A shortcoming of prior art sprayforming methods and apparatus is found in the wastage usually incurred in the form of relatively thin overspray layers of metal deposited on the edges of the descending plume of atomized particles projected onto a substrate material. These thin layers are a natural result of the generally Gaussian-shape curve of a cross-section of the generally conically-shaped particle plume, wherein is the particle depositions is more concentrated along the edge of the cone. The build-up is greater in the direction of the substrate motion. The lateral edges of the deposit may not be useful as a product and such wastage may amount to 15% or more of the deposited material.